This invention relates to a process and catalyst for reducing the aromatics and olefins content of hydrocarbon distillate products. More particularly, this process relates to an improved catalytic hydrogenation process and catalyst wherein the catalyst comprises platinum and palladium incorporated onto a support comprising zeolite Y and sodium.
For the purpose of the present invention, the term "hydrogenation" is intended to be synonymous with the terms "hydrotreating" and "hydroprocessing," and involves the conversion of hydrocarbons at operating conditions selected to effect a chemical consumption of hydrogen. Included within the processes intended to be encompassed by the term hydrogenation are aromatic hydrogenation, dearomatization, ring-opening, hydrorefining (for nitrogen removal and olefin saturation), and desulfurization (often included in hydrorefining). These processes are all hydrogenconsuming and generally exothermic in nature. For the purpose of the present invention, distillate hydrogenation does not include distillate hydrocracking which is defined as a process wherein at least 15% by weight of the distillate feedstock boiling above 430.degree. F. is converted to products boiling below 430.degree. F.
Petroleum refiners are now facing the scenario of providing distillate fuels, boiling in the range of from about 150.degree. F. to about 700.degree. F., with substantially reduced sulfur and aromatics contents. Sulfur removal is relatively well defined, and at constant pressure and adequate hydrogen supply, is generally a function of catalyst and temperature.
Aromatics removal presents a substantially more difficult challenge. Aromatics removal is generally a function of pressure, temperature, catalyst, and the interaction of these variables on the chemistry and thermodynamic equilibria of the dearomatization reaction. The dearomatization process is further complicated by the wide variances in the aromatics content of the various distillate component streams comprising the hydrogenation process feedstock, the dynamic nature of the flowrates of the various distillate component streams, and the particular mix of mono-aromatics and polycyclic aromatics comprising the distillate component streams.
The criteria for measuring aromatics compliance can pose additional obstacles to aromatics removal processes. The test for measuring aromatics compliance can be, in some regions, the FIA aromatics test (ASTM D1319), which classifies mono-aromatics and polycyclic aromatics equally as "aromatics." Hydrogenation to mono-aromatics is substantially less difficult than saturation of the final ring due to the resonance stabilization of the mono-aromatic ring. Due to these compliance requirements, hydrogenation to mono-aromatics is inadequate. Dearomatization objectives may not be met until a sufficient amount of the polycyclic aromatics and mono-aromatics are fully converted to saturated hydrocarbons.
While dearomatization may require a considerable capital investment on the part of most refiners, dearomatization can provide ancillary benefits. Distillate aromatics content is inextricably related to cetane number, the accepted measure of diesel fuel quality. The cetane number is highly dependent on the paraffinicity of molecular structures, whether they are straight-chain or alkyl attachments to rings. A distillate stream which comprises mostly aromatic rings with few or no alkyl-side chains generally is of lower cetane quality material while a highly paraffinic stream is generally of higher cetane quality.
Dearomatization of refinery distillate streams can increase the volume yield of distillate products. Aromatic distillate components are generally lower in gravity than their similarly boiling paraffinic counterparts. Saturation of aromatic rings can convert these lower API gravity aromatic components to higher API gravity saturated components and expand the volume yield of distillate product.
Dearomatization of refinery distillate streams can also provide increased desulfurization and denitrogenation beyond ordinary levels attendant to distillate hydrogenation processes. Processes for the dearomatization of refinery distillate streams can comprise the construction of a new dearomatization facility, the addition of a second-stage dearomatization step to an existing distillate hydrogenation facility, or other processing options upstream of distillate hydrogenation or at the hydrogenation facility proper. These dearomatization steps can further reduce the nitrogen and sulfur concentrations of the distillate component and product streams, thus reducing desulfurization and denitrogenation catalyst and temperature requirements in existing distillate hydrogenation facilities designed primarily for hydrorefining. Reduced distillate sulfur and nitrogen concentrations can additionally increase the value of these streams for use as blending stocks to sulfur-constrained liquid fuel systems and as fluid catalytic cracking unit (FCC) feed.
While distillate dearomatization can provide cetane number improvement, volume expansion, and additional desulfurization and denitrogenation, the process has seldom been attractive in view of the large capital costs and the fact that many refiners have not reached distillate cetane limitations. Now that legislation exists and further legislation is being considered to mandate substantial reductions in distillate aromatics content, the demand for distillate dearomatization processes is now being largely determined by the incentive to continue marketing distillates.
The use of zeolite Y in catalyst supports for hydrogenation has met with limited success and is commercially rare in distillate dearomatization. Zeolite Y, and zeolite supports in general, have not been commonly used in hydrogenation processes because the crystal structure of the zeolite, in combination with common commercial hydrogenation metals, such as nickel, molybdenum, and cobalt, generally provides lower desulfurization activity, has a tendency to promote undesired hydrocracking reactions, and can be prone to early deactivation.
Hydrocracking processes utilizing catalyst supports comprising zeolite Y have been the subject of several patents. U.S. Pat. Nos. 3,197,398, 4,104,152, and 4,202,758 are particularly directed towards processes for hydrocracking distillate and gas oil streams.
U.S. Pat. No. 3,197,398 to Young, discloses a distillate and gas oil hydrocracking process using a catalyst comprising a group VIII metal (IUPAC) such as palladium on a crystalline alumino-silicate support such as zeolite Y or mordenite. The hydrocracking process and catalyst are designed to convert high-boiling mineral oil feedstocks to lower boiling products such as gasoline. Hydrocracking reactions are not desired and are avoided in the hydrogenation process and catalyst of the present invention because hydrocracking reduces liquid product yield, increases undesirable light gas make, increases catalyst deactivation rates, and reduces distillate product cetane numbers.
U.S. Pat. Nos. 3,736,252, 3,773,654, 3,943,053, 3,969,222, 4,014,783, 4,070,272, 4,610,779, and 4,960,505 are particularly directed towards processes for hydrogenating distillate fuels.
U.S. Pat. No. 4,610,779 to Markley et al. discloses a distillate hydrogenation process using a catalyst comprising a group VIII metal and zeolite Y. The process requires contacting a feedstock containing organic nitrogen compounds with the catalyst wherein the catalyst is partially deactivated. The activity of the catalyst is subsequently restored through a high temperature hydrogen treatment step. The hydrogenation process of Markley et al. provides adequate hydrogenation activity, but can tend to cause hydrocracking of the distillate feedstock and is not particularly resistant to catalyst deactivation. The ammonia addition and high temperature hydrogen treatment steps are also generally undesirable operating steps.
U.S. Pat. No. 4,960,505 to Minderhoud et al. discloses a distillate hydrogenation process using a catalyst comprising at least one Group VIII noble metal on a support having a unit cell size between 24.20 .ANG. and 24.30 .ANG. and a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of at least 25. The process limits hydrocracking of the final product to less than 50 wt % of the feedstock material boiling between the 90 wt % and Final Boiling Point of the feedstock. While the hydrogenation process of Minderhoud et al. can provide reduced hydrocracking over prior art processes such as that disclosed in Markley et al., the process provides average hydrogenation activity and is not particularly resistant to catalyst deactivation.
The use of metal mixtures on a catalyst support has also been the subject of research. (See P. N. Rylander, Catalytic Hydrogenation over Platinum Metals, Academic Press, New York 1967.) Rylander teaches that two platinum metal catalysts, when used together, can give better rates or better yields than either catalyst individually. However, except for certain selected examples, there seems to be no way of predicting when mixtures of catalysts will prove advantageous. A useful guide as to the probable effectiveness of coprecipitated metal catalysts, is the performance of a mechanical mixture of the two metals. (See Rylander, at pages 9-11.)
U.S. Pat. No. 3,943,053 to Kovach et al. discloses a hydrogenation process using a catalyst comprising a particular mixture of platinum and palladium on an inert oxide support such as beta, eta, or gamma alumina. The process provides gasoline and distillate hydrogenation, but with limited hydrogenation activity. The process avoids use of silica-alumina supports since use of silica-alumina in gasoline service can result in the conversion of high octane benzene into substantially lower octane cyclohexane.
We have surprisingly found that catalysts and processes having a catalyst incorporating metal mixtures of platinum and palladium onto a support comprising zeolite Y combined with a particularly targeted concentration of sodium, result in substantially improved hydrogenation compared to prior art hydrogenation processes including processes having a catalyst incorporating platinum and/or palladium on a support comprising zeolite Y with lower or higher than the particularly targeted sodium levels. We have also found that catalysts having sodium concentrations above the particularly targeted concentration provide similarly inferior performance. The distillate hydrogenation catalyst in accordance with the principles of the present invention is also more resistant to catalyst deactivation than the prior art catalysts, resulting in longer operating facility run lengths before catalyst replacement.
We have also found that processes having a catalyst incorporating metallic mixtures of platinum and palladium on a support comprising zeolite Y with a particularly targeted concentration of sodium, result in substantially improved hydrogenation compared to processes having a catalyst having platinum or palladium incorporated onto the same support alone. This particular synergy is more profound (and in contradistinction to the teachings of Rylander) since physical mixtures of platinum and palladium on a support comprising zeolite Y with the same particularly targeted concentration of sodium have been shown not to provide improved hydrogenation.
It is therefore an object of the present invention to provide a process and catalyst that provide improved distillate aromatics saturation.
It is an object of the present invention to provide a process and catalyst that provide improved distillate desulfurization and denitrogenation.
It is an object of the present invention to provide a process and catalyst that increase distillate cetane number.
It is an object of the present invention to provide a process and catalyst that expand the volume of the distillate feedstock.
Other objects appear herein.